Apparatus For Offshore Transfer Of Fluid

ABSTRACT

Apparatus is described which is suitable for effecting the offshore loading and offloading of fluids, such as liquefied natural gas. The apparatus comprises a plurality of rotatable members ( 3,4,5 ) and a fluid transfer system suitable for connecting the apparatus to a fluid source or a fluid store. The apparatus is mounted on a base ( 2 ) and the fluid transfer system is mounted on or coupled to one of the rotatable members ( 3,4,5 ). In use, rotation of the rotatable members ( 3,4,5 ) relative to each other is controlled so as to maintain the fluid transfer system is a predetermined orientation relative to the base ( 2 ).

The present application relates to apparatus suitable for effecting theloading and offloading of fluids, such as liquefied natural gas.

Various methods have been developed to make offshore loading andoffloading of liquefied natural gas (LNG) a practical proposition. Sucha method is known from OTC paper 15301 (prepared for presentation at the2003 Offshore Technology Conference held in Houston, Tex., U.S.A., 5-8May 2003, and entitled “Offshore Transfer Re-Gasification and Salt DomeStorage of LNG”). This paper describes a single-point mooring systemsuitable for the mooring of an LNG carrier vessel. The paper furtherdescribes apparatus for discharging the LNG through the existingmid-ships manifold. The discharging apparatus comprises a rotatablymounted rigid arm which carries a standard fluid transfer system. Thefluid transfer system consists of three pipe-in-pipe (PIP) lines; two ofthe lines are dedicated to the transfer of LNG and the third line isdedicated to vapour return. The fluid transfer system is hingedlycoupled to the rigid arm to allow the arm to weathervane and pitch toaccommodate changes in movement between the vessel and platform.

When offloading from the mid-ship manifold of a vessel moored at asingle-point mooring structure, the structure may surge backwards andforwards under the influence of wave and wind loads. To accommodate thismovement, the rigid arm of the transfer apparatus described in theabove-referenced OTC paper would require a large reach capable ofcovering the full range of surge motions of the vessel, including thosewhich may occur after failure of a mooring hawser and any furthermovements which take place in the time it takes to shut transfer valvesbefore a physical disconnect can be performed.

With certain types of loading equipments, such as steel loading arms orflexible hoses, these large surge motions result in large angularoffsets between the extremities of such loading arms and hoses and thisposes large strains on their constituent components.

Moreover, since physical spacing between loading arms and hoses islimited, a certain “shadowing” occurs between the arms and hoses andhence, during a disconnect operation, clashing between mechanicalcomponents becomes unavoidable.

The present application, at least in preferred embodiments, attempts toaddress at least some of the problems outlined above.

Viewed from a first aspect, the present application relates to anapparatus suitable for the offshore transfer of fluids, the apparatuscomprising a plurality of rotatable members, and a fluid transfer systemsuitable for connecting the apparatus to a fluid source or a fluidstore; the apparatus being mountable on a base and said fluid transfersystem being mounted on or coupled to one of said rotatable members;wherein, in use, the rotation of the rotatable members relative to eachother is controlled so as to maintain the fluid transfer system in apre-determined orientation relative to the base.

The apparatus of the present invention advantageously creates a stableplatform on which the fluid transfer system may be mounted. The fluidtransfer system may comprise hard loading arms or flexible hoses capableof spanning the entire range of surge motions such that the loading armsor hoses themselves can be operated with virtually zero angular offsetduring large surge motions. Clearly, this arrangement provides improvedfunctionality and allows the fluid transfer system to operate withoutclashes occurring during the critical phases of any disconnectoperation.

The rotatable members are preferably each rotatable about a respectiveaxis. The axes are preferably parallel and, in use, the fluid transfersystem preferably undergoes translation in a plane substantiallyperpendicular to said axes.

The apparatus preferably comprises first, second and third rotatablemembers. The second rotatable member is preferably mounted on the firstrotatable member; and the third rotatable member is preferably mountedon the second rotatable member. The first rotatable member is preferablyrotatably mounted on the base which may in turn be fixedly mounted, forexample on a single point mooring structure. The base may in fact formpart of the single point mooring structure. The fluid transfer system ispreferably mounted on the third rotatable member. Of course, theapparatus may comprise 2, 4, 5, 6 or more rotatable members inalternative embodiments.

Preferably, the first member is rotatable about a first axis, the secondmember rotatable about a second axis and the third member rotatableabout a third axis. The first and second axes are preferably offset fromeach other; and the second and third axes are preferably also offsetfrom each other.

The apparatus thereby allows the fluid transfer system to undergotranslation relative to the base and may, therefore, accommodaterelative movement between a vessel and the fluid transfer system atleast in one plane.

The first, second and third axes are preferably all arrangedsubstantially vertically to facilitate translation of the fluid transfersystem in a substantially horizontal plane.

The apparatus is preferably configured such that when the first elementis rotated about the first axis through an angle of α°, the secondmember is rotated about a second axis through an angle of 2α° (i.e. thesecond member undergoes angular rotation twice that of the firstmember). The first member is preferably rotated in the first directionand the second member in a second direction, the first and seconddirections being opposite to each other.

The apparatus is preferably further configured such that, when the firstelement is rotated about said first axis through an angle of α°, thethird member is also rotated through an angle of α° (i.e. the angularrotation of the first and third members is the same). The first andthird members preferably rotate in the same direction.

This configuration of the first, second and third rotatable members mayallow the third member, and consequently the fluid transfer system, tobe maintained in a pre-determined orientation relative to the base, evenwhen the first and second members are rotated.

The fluid transfer system is preferably translated along a straight linewhilst the pre-determined orientation thereof is maintained. Thedistance between the first axis and the second axis is preferably thesame as the distance between the second axis and the third axis toenable the fluid transfer system readily to be translated along astraight line. It is, of course, possible to implement translation alonga straight line in arrangements where these distances are different,provided different angular rotations of the first, second and thirdrotatable members are implemented. The present application is intendedalso to cover these alternative arrangements.

The first, second and/or third rotatable members may be rotated byfirst, second and/or third actuators (such as electric motors orhydraulic cylinders) respectively. Preferably, however, the first memberis rotated by an actuator and first and second linkage assemblies affectrotation of the second and third rotatable members respectively.

The apparatus may further include means for determining the angularrotation of each rotatable member, to allow for control of theapparatus.

The fluid system transfer may comprise at least one conduit supported onfirst and second hard (i.e. non-flexible) support arms. Alternatively,the fluid transfer system may comprise at least one flexible hose.

A conduit is preferably provided which defines a fluid pathway for thetransfer of fluids through said rotatable members. Fluids to betransferred using the apparatus of the present invention may then bepassed from the fluid transfer system through the interior of therotatable members. Alternatively, one or more conduits may be providedto the exterior of the rotatable members.

The base on which the first rotatable member is rotatably mounted maybe, for example, a column. The column may be provided on a single pointmooring structure, or on a LNG carrier vessel.

Two preferred embodiments of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows the transfer apparatus of the present invention mounted ona single point mooring structure;

FIG. 2 shows the apparatus of FIG. 1 from a different perspective;

FIG. 3 shows an end view of the apparatus of the present invention; and

FIG. 4 shows a second embodiment of the present invention.

A single point mooring (SPM) structure 1 of the type described in OTCpaper 15301 is shown by way of example in FIG. 1, adjacent to tankervessel V. Transfer apparatus in accordance with the present invention isprovided on a column 2 at the aft-end of the SPM structure 1. Thetransfer apparatus comprises a first rotatable member 3 which is mountedon the column 2. A second rotatable member 4 is mounted on the firstrotatable member 3; and a third rotatable member 5 is mounted on thesecond rotatable member 4.

As shown in FIG. 2, a fluid transfer system 6 is provided on the thirdrotatable member 5. The fluid transfer system 6 comprises four hardconduits 7-10, each of which are carried by a support arm 11-14.

The first rotatable member 3 is rotated by an electric motor. It will,of course, be appreciated that other actuation means, such as ahydraulic cylinder, may be implemented to effect rotation of the firstrotatable member 3. The transfer apparatus further comprises a linkagebar system 15 for effecting angular rotation of the second and thirdrotable members 4, 5. The implementation of a linkage bar system 15advantageously removes the need to provide additional actuation meansand may thereby reduce the cost of the transfer apparatus.

The first rotatable member 3 is rotated about a first axis A; the secondrotatable member 4 is rotated about a second axis B; and the thirdrotatable member 5 is rotated about a third axis C. The first, secondand third axis A, B, C are arranged substantially vertically such thatthe first, second and third rotatable members 3, 4, 5 each rotate inrespective substantially horizontal planes. The distance between thefirst axis A and the second axis B is the same as the distance betweenthe second axis B and the third axis C.

The linkage bar assembly 15 is such that when the first rotatable member3 undergoes an angular rotation of α° relative to the column 2, thesecond rotatable member 4 undergoes an angular rotation of 2α° relativeto the first rotatable member 3. For example, if the first rotatablemember 3 is rotated through 30° relative to a reference plane, thesecond rotatable member 4 undergoes an angular rotation of 60° relativeto the first rotatable member 3.

The linkage bar system 15 is arranged such that rotation of the firstrotatable member 3 through an angle of α° results in a correspondingrotation of the third rotatable member 5 through an angle of α° relativeto the second rotatable member 4.

The linkage bar assembly 15 is arranged such that the first and thirdrotatable members 3, 5 rotate in the same direction, whereas the secondrotatable member 4 rotates in the opposite direction. Thus, in thearrangement shown in FIGS. 1-3, rotation of the first rotatable member 3in a clockwise direction (viewed from above) causes the third rotatablemember 5 to rotate in a clockwise direction, and the second rotatablemember 4 to rotate in an anti-clockwise direction.

The net result of the arrangement of the first, second and thirdrotatable members 3, 4, 5 is that the fluid transfer system 6 undergoestranslations along a horizontal axis. Advantageously, the transferapparatus is arranged such that the fluid transfer system may undergotranslation along a longitudinal axis parallel to an adjacent edge ofthe SPM structure 1. Thus, the fluid transfer system 6 may translate inthe surge direction of the vessel carrying the liquefied natural gaswithout getting closer or further away from the manifold provided on thevessel.

It will be appreciated that the arrangement of the transfer apparatus issuch that the orientation of the fluid transfer system 6 relative to theSPM structure 1 is maintained constant as it undergoes translation. Thisfunctionality is especially desirable as the stresses applied to thesupporting arm 11-14 may be reduced as the fluid transfer system 6 maybe maintained in an orientation perpendicular to the vessel.

Moreover, the longitudinal position of the fluid transfer system 6 maybe maintained constant relative to the LNG carrier vessel by effectingappropriate rotation of the first, second and third rotatable members 3,4, 5. This may further reduce or minimise side-loads on the fluidtransfer system 6.

The support arms 11-14 are provided with an elbow joint to accommodatetransverse movement of the LNG carrier vessel relative to the SPMstructure 1.

As shown in FIG. 3, the transfer apparatus is provided with a series offluid swivels 16 to allow the transfer of fluid through the transferapparatus irrespective of the angular orientation of the first, secondand third rotatable members 3, 4, 5.

In order to monitor the motions of the LNG carrier vessel carrying theliquefied natural gas, an optical or other tracking system may beprovided. This tracking system preferably provides continuousinformation as to the location of the vessel relative to the SPMstructure 1 and allows the angular orientation of the first, second andthird rotatable members 3, 4, 5 to be controlled to maintain thelongitudinal position of the fluid transfer system 6 constant relativeto the vessel.

A second embodiment of the transfer apparatus is shown in FIG. 4. Inthis embodiment, like reference numerals have been used for likecomponents. The apparatus of this embodiment is modified insofar as thehard loading arms 11-14 have been replaced by a set of catenary hoses 17mounted on the underside of the third rotatable member 5.

To allow the catenary hoses to be suspended under the transferapparatus, the third rotatable member 5 is rotatably mounted on theunderside of the second rotatable member 4, which in turn is mounted onthe underside of the first rotatable member 3.

The third rotatable member 5 is also provided with a support mechanism18 for carrying the fluid transfer system 6.

The angular control of the first, second and third rotatable members 3,4, 5 is generally the same as for the first embodiment such that thefluid transfer system 6 may translate along an axis parallel to thelongitudinal axis of the SPM structure 1.

The support member 18 advantageously allows for transverse movement ofthe vessel relative to the SPM structure 1 in the same way as thismotion was accommodated by the hard loading arms 11-14 of the firstembodiment.

It will be noted that in the second embodiment, the distance between thefirst and second axis A, B is not the same as the distance between thesecond and third axis B, C. The relative angular rotation of the first,second and third members 3, 4, 5 is modified accordingly to ensure thatthe fluid transfer system 6 is maintained in a pre-determinedorientation as it undergoes translation.

In certain cases, the assembly of the first, second and third rotatablemembers 3, 4, 5 is more optimally located well above the water level,for example to allow sufficient space for catenary type hoses, as shownin FIG. 4. In other applications, such as those with hard loading arms11-14, as shown in FIGS. 1-3, the first, second and third rotatablemembers 3, 4, 5 are better placed closer to the water line.

1. Apparatus suitable for the offshore transfer of fluids, the apparatus being mountable on a base and comprising first, second and third rotatable members, and a fluid transfer system suitable for connecting the apparatus to a fluid source or a fluid store, second rotatable member, and the fluid transfer system being mounted on said third rotatable member, wherein the first member is rotatable about a first axis, the second member is rotatable about a second axis, and the third member is rotatable about a third axis, the axes being substantially vertical and parallel, the first and second axes being offset from each other and the second and third axes being offset from each other, and wherein the apparatus is configured such that when the first member is rotated about first axis through an angle of α, the second member is rotated about said second axis through an angle of 2α°; whereby in use, the fluid transfer system is maintained in a predetermined orientation relative to the base.
 2. Apparatus as claimed in claim 1, wherein the fluid transfer system, in use, undergoes translation in a plane substantially perpendicular to said axes.
 3. (canceled)
 4. Apparatus as claimed in claim 1, wherein the first rotable member is rotatably mountable on the base. 5-7. (canceled)
 8. Apparatus as claimed in claim 1, wherein when said first member rotates in a first direction the second member rotates in a second direction, the first and second directions being opposite to each other.
 9. Apparatus as claimed in claim 8, wherein the apparatus is configured such that when the first element is rotated about said first axis through an angle of α°, the third member is rotated about said third axis through an angle of α°.
 10. Apparatus as claimed in claim 9, wherein the apparatus is configured such that said first and third members rotate in the same direction.
 11. Apparatus as claimed in claim 1, wherein the distance between the first axis and the second axis is the same as the distance between the second axis and the third axis.
 12. Apparatus as claimed in claim 1, further comprising a first actuator for rotating said first rotatable member.
 13. Apparatus as claimed in claim 1, further comprising a second actuator for rotating said second rotatable member relative to said first rotatable member.
 14. Apparatus as claimed in claim 1, further comprising a third actuator for rotating said third rotatable member relative to said second rotatable member.
 15. Apparatus as claimed in claim 1, further comprising a first linkage assembly for effecting rotation of the second rotatable member relative to the first rotatable member.
 16. Apparatus as claimed in claim 11, further comprising a second linkage assembly for effecting rotation of the third rotatable member relative to the second rotatable member.
 17. Apparatus as claimed in claim 1, further comprising means for determining the angular rotation of each rotatable member. 18-23. (canceled) 